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FIELD EMISSION MITIGATION VIA IN-SITU PLASMA PROCESSING IN 1.3 GIGAHERTZ 9-CELL LCLS-II CAVITIES
Title
FIELD EMISSION MITIGATION VIA IN-SITU PLASMA PROCESSING IN 1.3 GIGAHERTZ 9-CELL LCLS-II CAVITIES
Creator
Giaccone, Bianca
Date
2021
Description
Field emission (FE) is one of the limiting factors in superconducting radiofrequency cavities' performance. It is known that even a few...
Show moreField emission (FE) is one of the limiting factors in superconducting radiofrequency cavities' performance. It is known that even a few monolayers of surface adsorbed contaminants can lower the niobium work function and increase the FE. In order to address the field emission that may arise once the accelerator is already assembled, it was decided to develop plasma processing for the Linac Coherent Light Source II, a method to mitigate field emission in-situ. Starting from Doleans's successful experience with plasma processing for high beta cavities, Fermi National Accelerator Laboratory is developing plasma cleaning for TESLA shaped 1.3 GHz 9-cell cavities. A new method of ignition based on the higher order modes and couplers was developed, along with a detection procedure that allows to identify the location of the plasma inside the cavity. In this work are presented the results of plasma processing applied to 1.3 GHz cavities, both single-cell and 9-cells. The cavities were contaminated with multiple sources, naturally or artificially, and their performance was measured through cryogenic RF tests before and after plasma cleaning. These experiments proved that plasma processing successfully removed hydrocarbon-related field emission from cavities artificially contaminated, but also from a cavity with natural and unknown FE source. In some cases of more extreme contamination through vacuum failure simulation conducted in air (not in a cleanroom), plasma processing was not able to recover the cavity's performance. An ongoing analysis of the cavity contaminants is presented here, explaining the reason why some contaminated cavities showed little improvement after plasma processing. A microscopic study of the effect of plasma processing on the niobium surface is also presented. Niobium samples prepared with different surface treatments were analyzed using X-ray photoelectron spectroscopy, scanning electron microscopy and energy-dispersive X-ray spectroscopy. The samples were subjected to plasma processing and analyzed again, in order to draw a comparison and identify possible surface changes caused by the reactive oxygen contained in the glow discharge. The samples were prepared with different surface treatments in order to understand if plasma processing may affect them differently. This study showed a possible increase in the oxide thickness after plasma processing and a reduction of the energy difference between the pentoxide and the metal peaks. In preparation for this study, the near-surface region of one niobium sample was investigated with X-ray photoelectron spectroscopy at various steps of sputtering and subsequent oxide regrowth in air. The results showed that the majority of the oxide is composed of Nb2O5, however, the presence of two suboxides (NbO, NbO2) is observed, plus an additional peak (attributed to Nb2O) measured both during sputtering and oxide regrowth.
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